As you shop for plywood for your woodworking projects, you will need to consider not only the various grading classifications, but also the way in which plywood panels are sized. Shopping at a big-box home center might lead you to believe it's an easy matter of choosing 4 x 8 sheets in thicknesses of 1/4-inch, 1/2-inch, or 3/4-inch, but it is not that simple—especially if you shop at a specialty lumberyard or woodworking supply outlet.
Most plywood products suitable for use in woodworking projects are marketed as 1/4-inch, 1/2-inch, 5/8-inch, or 3/4-inch thicknesses, but it's important to realize that these are nominal sizes, not actual dimensions. Just as construction lumber nominal sizes differ from actual dimensions, the same is true of plywood sheet goods:
|NOMINAL VS. ACTUAL SIZES|
Specialty woodworking shops may carry plywood in additional thicknesses, such as 1/8-inch (3/32) panels for use as a facing material.
Although with 1/32-inch difference doesn't seem like much—and it doesn't make much difference in rough construction work where the tolerances are less demanding—it can become an issue in precision woodworking. For instance, if a woodworker is building a bookshelf where a "3/4-inch" plywood shelf is placed into a dado slot cut with a 3/4-inch router bit, there will be a very noticeable 1/32-inch gap, and the shelf will feel a bit sloppy in the dado. To combat this, the dado should be cut at 23/32 inch to ensure a proper fit. All fine cabinetry and woodworking projects need to take into account the differences between nominal and actual measurements of plywood and other wood boards.
Many retailers now label plywood products with the actual thicknesses, as well as the nominal thicknesses.
Plywood Sheet Dimensions
Big-box home improvement centers generally sell sanded pine or hardwood-faced plywood suitable for woodworking projects in standard 4 x 8-ft. sheets. There also may be 4 x 12-ft. sheets available by special order (these larger sheets will generally be in-stock at major lumberyards). Home improvement centers may also sell "project panel" plywood pieces in 2 x 2-ft., 2 x 4-ft. or 4 x 4-ft. sizes. These project panels are usually sawn to size from large panels by the store to meet local demand.
Specialty woodworking stores often more variety (and also higher prices), selling a variety of hardwood-faced plywood panels in sizes ranging from 12 x 30 to 24 x 48 inches. Baltic birch plywood panels (a type of plywood consisting of birch veneers throughout) are sometimes sold as 5 x 5-ft. panels.
Why Nominal Sizes Differ From Actual Sizes
In framing lumber, the difference between the nominal size (how a board is labeled) vs. its actual size comes about because of the planing and drying process, which shrinks a board from the dimensions it had when it was originally milled. For example, a stud-grade pine board is initially milled to a precise 2 x 4-inch dimension, but the subsequent drying and planing process shrinks the board to about 1 1/2 x 3 1/2 inches in size.
With hardwood boards sold in stores, the difference in nominal vs. actual dimensions is usually about 1/16 inch; it is the result of the planing process used to finish the board faces. Woodworkers need to keep this in mind when using solid hardwoods in their projects.
With plywood products, the difference between nominal and actual thickness comes about because of the sanding process used to finish the face of the panel after the layers have been glued and dried. This sanding typically removes 1/32 inch from the face of the plywood.